Analysis of carbon cycle interference mechanisms in karst rivers under the combined influence of acid mine drainage and multi-source anthropogenic pollution

Abstract

Acid mine drainage (AMD) triggers complex environmental chain reactions, including hydrochemical imbalance, heavy metal accumulation, and carbon cycle disruption. In karst regions, the coupled surface-subsurface structure accelerates pollutant migration and environmental risks. AMD-induced dissolved inorganic carbon (DIC) loss and enhanced carbonate dissolution weaken the karst carbon sink. To clarify the AMD effects on carbon cycling in karst rivers, this study investigated the Yudong River Basin in Kaili City, Guizhou Province, China. Based on multi-temporal sampling data collected from 2024 to 2025, combined with hydrochemical ions, DIC, and carbon isotopes (δ¹³C_DIC) analyses, we examined hydrochemical evolution and carbon cycling under AMD and anthropogenic influences. Results showed that the dominant hydrochemical type was SO₄²⁻·Ca·Mg-type water, indicating a shift from carbonate-driven to sulfate-driven weathering. AMD significantly increased SO₄²⁻, Ca²⁺, and Mg²⁺ concentrations, weakened the HCO₃⁻-dominated carbonate system, lowered pH, and enhanced partial pressure of carbon dioxide (pCO₂) and CO₂ emissions (FCO₂). The average pCO₂ and FCO₂ in AMD-affected reaches were approximately 9.94 and 9.37 times higher, respectively, than those in unaffected reaches, contributing up to approximately 63% of the basin's CO₂ emissions (25.57 Gg C yr⁻¹ km⁻²) and markedly reducing the basin's net carbon sequestration efficiency. Redundancy analysis showed that hydrochemical indicators explained 62.86% of the relative contribution to the carbon cycle, indicating a pronounced anthropogenic effect. Without significant DIC enrichment, sulfide oxidation and Fe³⁺ hydrolysis may jointly promote DIC conversion to CO_2(aq) in AMD source areas. During downstream dilution, enhanced CO₂ degassing led to δ¹³C_DIC enrichment, whereas under low-pH mixed pollution, carbonate dissolution, organic matter mineralization, and sulfate reduction may increase DIC and shift δ¹³C_DIC negatively. Overall, AMD is the primary driver of carbon cycle imbalance and increased carbon emissions in the karst river basin, while multiple anthropogenic pollution sources significantly alter carbon migration and transformation pathways.